The present invention relates to pyromellitic dianhydride polyamide acid coating compositions having improved shelf stability and method for making. More particularly, the present invention relates to the use of an organic silazane, such as hexamethyldisilazane, as a stabilizer for polyamide acid solutions of pyromellitic dianhydride.
Prior to the present invention, pyromellitic dianhydride (PMDA) was commonly used in the manufacture of polyamide acids in the form of a substantially anhydrous solution of the polyamide acid in dipolar aprotic solvent, such as N-methylpyrolidone. It is generally known, however, that polyamide acid solutions derived from PMDA suffer from unsatisfactory storage stability, as shown by T. Inaike et al., U.S. Pat. No. 4,535,105. In some instances, refrigeration has been found useful for improving the shelf stability of polyamide acids, where the polyamide acid was in the form of an aqueous dispersion useful for electrodeposition, as shown by Boldebuck, U.S. Pat. No. 3,767,550. In particular instances, ammonium salts of saturated monocarboxylic acids, or ammonium salts of aromatic monocarboxylic acids have been used during the formation of the polyamide acid to control the viscosity of the polyamide acid, such as shown by Boldebuck et al., U.S. Pat. No. 3,496,132. Bases, such as alkali metal hydroxides, organic bases such as guanidine or watersoluble organic amines such as pyridine, morpholine, triethylamine and ammonium salts also have been used to partially neutralize aqueous polyamide acid dispersions.
It would be desirable to make substantially anhydrous polyamide acid solutions which are convertible to polyimide films or as polyimide binders for making composites where the polyamide acid solutions can be stored over an extended period of time such as 30 days or more and thereafter converted to useful polyimide films or composite binders.
The present invention is based on the discovery that organic silazane, such as hexamethyldisilazane, can be incorporated into a solution of a pyromellitic dianhydride polyamide solution, to produce a polyamide acid solution having substantially improved shelf stability. It has been further found that polyamide acids in the form of copolymers of PMDA units, and units resulting from the condensation of bisphenol A dianhydride, as defined hereinafter, with appropriate aryldiamines also can be stabilized with effective amounts of the organic silazane.